The requirement to provide an alterable display of data or graphical representation in locations where no power is available, or is only intermittently available, has existed for some time. For example in cashless payment systems such as are used for vending and payphones, where magnetic or optical cards are used to store value, the credit remaining on the card can only be read during the time it remains inserted in a terminal device with display capability. At all other times the value and any other data on the card is invisible. Users of such cards would find it advantageous to be able to see at any time the remaining value of the card.
Developments of smart (integrated circuit) card technology for cashless payment and other applications requiring enhanced data storage capacity and security capability have highlighted the need to be able to display some of the variable data `stored electronically on the types of card commonly used in such applications or data from an external source. Requirements include; the need to write data in alignment with fixed characters printed on the card, for example a currency symbol; writing multiple fields of data accurately in relation to descriptors printed on the card, for example `remaining credit`, `last transaction`, `appointment date`, `service due date`; to be able to modify the display to reflect changes in the value of the data recorded on the card and to provide the display function on thin cards similar to the common credit card which typically carry no energy source. Applications of smart cards are being developed to extend use from credit and debit to stored value functions and the electronic purse concept where the user may need to refer to the remaining `value or values and other data on the card plus data from some other source in the absence of a terminal device with display capability. Other possible applications of non-volatile displays exist where costly permanent printing is periodically discarded and a new display installed. For example, product, price and bar code displays mounted on supermarket shelving units are from time to time required to be changed. Non-volatile re-writable display devices would offer a low cost, re-usable alternative.
There are known display materials which are responsive to magnetic fields or heat to reversibly produce contrasting light and dark areas such that with proper formatting any alpha or numeric character can be written or any graphical shape may be drawn with a suitable write mechanism. However, unlike electrically driven displays such as are typically found incorporated in battery operated calculators, remote controllers and similar devices, in which the display formatting framework is predetermined in manufacture, the magnetic field or heat sensitive display materials have no formatting framework and no integral means of self clearing and re-writing. A write mechanism must be provided, for convenience mounted in a card terminal or similar device, such that as the display is moved relative to the write mechanism, appropriate magnetic or heat stimuli are generated using electrical signals derived from stored data together with electrical power from the terminal device which may be either battery-or mains operated. The spatial formatting of characters to be written on the display is determined in a first dimension by the pitch of individual writing elements of the writing mechanism and in a second dimension by a measuring scale. Such scales may be part of the terminal device driven by frictional contact with the display material or display carrier. However such an arrangement has the disadvantage that card or carrier thickness variations cause variable frictional contact resulting in variable character formatting and positioning. A further disadvantage is that wear in the terminal devices will also cause variable formatting and positioning of characters. Yet a further disadvantage is the requirement for the terminal device to incorporate a large number of moving parts subject to wear and failure.
EP 0473403A2 published in 1992 discloses a card with an erasable re-writable medium used in apparatus which comprises an array of write heads and relies on drive rollers to position the card under the control of card positioning sensors which only detect the position of the card. There is no teaching of markings on the card to define the subsequent card location in the apparatus and the problems of writing to small or discrete write zones where precise positioning is necessary either to coincide with preprinted characters or to avoid destructive overlaps at edges of the fields. Evidently, reliance is placed on the provision of a relatively vast area of re-writable material covering the entire card face so the latter problem is not addressed.
WO 87/00945, published 1987, addresses the problem of transferring optical disk technology to a wallet size data card of 0.25 meg. capacity. The data is optically (re)written in minute, binary form by a 25 micron laser spot which cannot be confused with humanly legible characters. The references teaches reference position information on the recording strip, but only for guidance and definition of specific zones for different information, a well known disk formatting approach for defining data clusters, not to identify individually written humanly readable character locations.
GB 2161425A, published 1986, teaches a formatted data card which is not re-writable but only has provision for decrementing stored value sites so small (10.times.8 micron,) as to be effectively illegible, as the optical system follows a servo track and reads calibration marks. Reliance is placed on a grating system for interpolation between the calibration marks to adjust for card movement error in the environment of use showing that specific read/write sites are not sufficiently defined by markings on the card alone.
GB 2199684A, published 1988, teaches another system for writing and reading minute optical data on a formatted data card relating data records to control records which are placed on the card in an essentially arbitrary manner. The problem of accurately locating individual humanly legible characters is not addressed.
Although GB 2218041A, published 1989, teaches provision of humanly visible markings aligned with a graticule, they are not erasable or re-writable with different values but are irreversibly changed to indicate that only fixed value has been used. They are in only linear array and the concept of forming a two dimensional graphical character is not addressed.
In so far as they may be relevant, the disclosures of the above are incorporated herein by reference.